The present invention relates generally to induction heating of billets, and in particular, to simultaneous induction heating of multiple billets in a sequenced process.
A heated metal billet can be worked into a manufactured article by, for example, forging or die casting the heated billet. Ideally the billet is heated throughout its cross section to a substantially uniform temperature that is slightly below the melting point of the billet material for maximum workability of the billet. Uniformity of temperature throughout the billet material avoids the formation of isolated solid or molten regions within the billet that can result in deformities of the worked article. One method of heating and melting an electrically conductive billet, such as an aluminum billet, is by electric induction heating. In this method, a magnetic field generated by the flow of ac current in a coil placed around the axial length of the billet will heat the billet by magnetically coupling the field with the billet. The resulting magnetic field penetrates the billet and produces an eddy current in the billet, which heats the billet material. Some electrically conductive materials, such as aluminum based compositions, exhibit a relatively small degree of field penetration into the material. FIG. 1 illustrates the typical drop off in the effectiveness of heating a billet 11 by magnetic induction from field 90, which is shown diagrammatically as sample flux (dashed oval) lines for a field produced by an induction coil surrounding the axial length of billet 11. As illustrated by curve Iind in the Im versus rm graph in FIG. 1, the depth (or magnitude) of the induced eddy current, Im, in a billet having a radius, rm, rapidly decreases towards the axial center of the billet. Consequently effective induced eddy current (dashed horizontal lines in FIG. 1) heating of the billet is concentrated in the outer annular region of the billet, xcex94m, which is defined as the magnetically induced eddy current depth of penetration. Attempting to rapidly heat an aluminum billet throughout its entire thickness by induction to a generally uniform temperature will result in melting the outer annular region of the billet material before the required level of heat is reached at the center of the billet. Consequently the applied level of induced billet heating power must be limited. This can be accomplished either by maintaining a relatively low and constant induced heat energy (power multiplied by the applied time period) during the entire heating cycle for a billet, or by initially applying a high level of induced heat energy, followed by decreasing levels of induced heat energy over the entire heating cycle for a billet. As the outer volume of the billet is inductively heated, heat conducts into the center of the billet material. The process is particularly effective with a billet metal composition, such as an aluminum or magnesium based composition, which has a relatively high value of thermal conductivity. This process is sometimes described as heat xe2x80x9csoakingxe2x80x9d the billet, since the magnetically induced heat xe2x80x9csoaksxe2x80x9d to the interior of the billet by conduction of heat through the billet material.
Early prior art billet induction heating is disclosed in U.S. Pat. No. 3,535,485 (the 485 patent), titled Induction Heating Device for Heating a Succession of Elongated Workpieces. The 485 patent teaches sequential pushing of billets into two or more separate induction coils for heating so that heated billet production can be increased by sequencing an automated billet feeding mechanism 12 with the two or more separate induction coils. In this fashion, a billet in each of the two or more separate induction coils is heated to a different degree at any instant of time. The billet feeding mechanism 12 indexes to an induction coil with a fully heated billet and ejects the fully heated billet by pushing a non-heated billet into the induction coil. The 485 patent does not teach varying the induced heat energy, or staging induced heat energy sequentially among the two or more separate induction coils.
U.S. Pat. No. 4,307,278, titled Control Device for Parallel Induction Heating Coils teaches the use of a plurality of induction coils that are connected in parallel to a single power source. An elongated workpiece is heated in each of the coils. Induced heat energy in each workpiece is varied by mechanically adjusting the length of the coil based upon feedback from a temperature sensor so that uniform heating of the workpiece can be achieved.
Another known method of heating a billet is the use a carousel system in which a billet is sequentially transferred among induction heating coils. The coils are of varying configurations so that they induce progressively lower levels of energy to a billet as it is sequenced in the carousel system. The system can be used to simultaneously heat as many billets as there are induction coils in a sequenced process. For example in a vertically aligned carousel system, multiple vertically aligned and radially spaced billets sit on a carousel. A multiple coil assembly consisting of a sequence of induction coils arranged for inductive energy transfer is disposed above the billets. The multiple coil assembly can be lowered so that each coil surrounds a billet and transfers varying levels of inductive energy to the billets on the carousel. After a selected period of time, the multiple coil assembly is raised and the carousel with billets is indexed so that each billet moves to the next lower inductive energy coil. The fully heated billet that was last surrounded by the lowest inductive energy coil in the assembly is removed from the carousel and a non-heated billet is put in its place on the carousel to be surrounded by the highest inductive energy coil in the assembly to propagate the billet heating process. This method is disadvantageous in that the billets are vertically oriented and the outer volume of the billets, having been subjected to all of the induced heat energy, tend to sag by completion of the heating process for a billet. This method also requires moving the billets during the indexing process.
Therefore there is the need for apparatus and method of inductively heating a billet that minimizes deformation and handling of a billet during the heating process to a substantially uniform temperature that may be close to the melting temperature of the billet material.
In one aspect, the present invention is an apparatus for and method of inductively heating a plurality of billets. Each billet is surrounded by an induction coil. All of the induction coils can be connected to a single ac power supply in a circuit having an individual power switch between the supply and each coil. Output of the power supply can be kept at a constant level while the output is sequentially switched among each of the induction coils. Switched power scheduling to each coil is such that the power supply provides inductive power over progressively shorter time intervals, and hence, a progressively smaller amount of heating energy to each coil in the sequence during an applied power cycle. The current in each coil creates a magnetic field that couples with the billet in the coil and inductively heats the billet. During the power dwell time between the repetitive applications of power to a coil by the power switch, the induced heat conducts into the interior of the billet. With appropriate switched power scheduling among all the coils, billets are sequentially fully heated at the end of a billet heating cycle.
In another aspect, the present invention is an apparatus for and method of sequentially induction heating a plurality of billets. Each billet is inserted into a separate induction coil so that the axial length of the billet is substantially surrounded by the induction coil. At least one ac power supply is used to provide ac current sequentially to each of the induction coils for a variable time period in multiple power cycles. The power supply may optionally operate at a substantially constant magnitude of output power. The total number of power cycles is equal to the total number of induction coils. The variable time period during which ac current is supplied to an induction coil is progressively shorter in each power cycle. Each induction coil receives ac current for the same set of variable time periods over all of the power cycles, but in any particular power cycle, each induction coil receives ac current for different variable time periods. The ac current supplied to each induction coil is inductively coupled with the billet inserted in the coil, which inductively heats the billet. A billet is completely heated after it has been subjected to sequential induction heating for the total number of power cycles. The apparatus may optionally include a means for inserting a billet into an induction coil at the beginning of the power cycle wherein the induction coil is connected to the at least one ac power supply for the longest variable time period. Further the apparatus may optionally include a means for removing a billet from an induction coil after the completion of the total number of power cycles for the coil. Optionally a processor may be provided for sensing the surface temperature of each of the billets while it is being induction heated, and responsive to the sensing, the magnitude of the output power from the ac power supply or the time of the variable time periods may be adjusted to complete the induction heating of the billets.
Other aspects of the invention are set forth in this specification.